1. Field of the Invention
The present invention relates to a position detector for detecting a position of a movable member, in particular, a position detector for detecting a position of a movable member based on multiple signals.
2. Description of the Related Art
As an apparatus for measuring a distance of movement of an object, besides an incremental encoder for measuring a distance of relative movement, there is hitherto known an absolute encoder capable of detecting an absolute position.
Japanese Patent Application Laid-Open No. H08-304113 discloses a Vernir type absolute encoder. The Vernir type absolute encoder disclosed in Japanese Patent Application Laid-Open No. H08-304113 has a configuration including a main track and at least one sub track, which are formed by grid patterns having different pitches. Based on a slight shift between detected signals, which is generated due to a difference in pitch between the tracks, detection of an absolute position in a section within which relative shifts among the signals originating from the grid patterns show a single cycle can be performed by the Vernir type absolute encoder.
Japanese Patent Application Laid-Open No. H05-45151 discloses timing of switching to an absolute counting operation in the Vernier type absolute encoder. The Vernier type absolute encoder has a configuration in which tracks are formed at smaller pitches and at larger pitches. As the timing of switching to the absolute counting operation based on the tracks described above, when a speed is lowered to a speed at which erroneous detection does not occur in an incremental measurement at the smaller pitches, an operation of the Vernier type absolute encoder is switched to the absolute counting operation.
However, the absolute encoder disclosed in Japanese Patent Application Laid-Open No. H08-304113 has the following problems. When the absolute position is calculated from the signals of the multiple tracks, synchronism of the signals of the multiple tracks of the absolute encoder is generally required to be ensured. This is because the calculation of the absolute position presupposes that the signals of the multiple tracks are signals generated when the position of an object to be measured is identical or within an allowable range. In the case where the signals of the multiple tracks are detected in time division, the signals of the multiple tracks are signals corresponding to different positions of the object to be measured when the object to be measured is in a moving state. Therefore, the above-mentioned presupposition is not established. As a result, there arises a problem in that a precise absolute position cannot be calculated. On the other hand, there is a method using a sample-hold circuit to ensure the synchronism of the signals of the multiple tracks. However, the method described above has a problem in that costs are increased due to an additional circuit or extra time is required to hold the signals.
Moreover, the absolute encoder disclosed in Japanese Patent Application Laid-Open No. H05-45151 has the following problem. Japanese Patent Application Laid-Open No. H05-45151 only examines erroneous detection in the incremental measurement at the smaller pitches as erroneous detection because the detection of the signals of the multiple tracks in time division is not supposed. In order to calculate a precise absolute position, however, it is important for the signals of the multiple tracks to be signals obtained when the object to be measured is located within the allowable range. Therefore, even when there is no erroneous detection in the incremental measurement at the smaller pitches, it cannot be said that the synchronism of the signals of the multiple tracks is ensured. Therefore, a precise absolute position cannot be calculated. Specifically, if the calculation of the absolute position is determined based on an erroneous detection of the incremental measurement at the smaller pitches, there arises a problem in that a wrong absolute position is disadvantageously calculated.